Effects Of High-Temperature Oxidation And Strengthening On The Friction And Wear Properties Of Cu-based Friction Materials

Brake pad is the core component to ensure the stable operation of the high-speed train braking system.Cu-based brake pad is widely used as a friction material in high-speed train braking system with excellent performance and economy.A large amount of friction heat is generated when the train is under emergency braking,which leads to oxidation of the Cu-based brake pad and significantly affects its mechanical and tribological properties.Therefore,it is of great significance to further study the effect of high temperature oxidation on the action mechanism and performance of Cu-based brake pad,and carry out targeted strengthening design,to improve the braking performance of Cu-based brake pad at high speed braking.In this paper,the Cu-based brake pad for high speed train was prepared by powder metallurgy process,the high-temperature oxidation mechanism of Cu-based brake pad was investigated,and then the effects of pre-oxidation treatment on the microstructure,mechanical properties and tribological properties of the materials were investigated.Then AlCoCrFeNi high-entropy alloy was introduced as the reinforcement component to prepare the Cu-based friction materials reinforced with AlCoCrFeNi HEA,and then the braking performance of Cu-based friction materials reinforced with HEA was t investigated.The details are as follows:(1)The oxidation behavior of the Cu-based brake pad is investigated via isothermal oxidation at 300,400,500,600 and 700℃.The results show that the oxidation of the Cu-based brake pad presents multiple stages.The combination of the oxidation of Cu and Fe and the oxygen diffusion controls the oxidation process in the earlier stage,while the oxidation of graphite plays a more important role in the later stages at above 500℃.(2)The microstructure evolution of the Cu-based brake pad oxidation at 300,400,500,600 and 700℃ is evaluated.The results show that the oxidation of copper first forms Cu2O nanoclusters,then CuO nanowires,and finally fine and coarse equiaxed grains.The rise in temperature promotes the growth and densification of Fe2O3 nanosheets,which grow on the Fe3O4 layer.However,Fe oxides are gradually covered by Cu oxides because of the larger volume expansion of Cu oxides.The connected pores formed by the graphite burn-off provide oxygen diffusion channels for internal oxidation.(3)Cu-based brake pads were pre-oxidized at 25～600℃ to simulate the oxidation process.The results show that,as the oxidation temperature rises,surface oxides change from Cu2O+Fe3O4 to CuO+Fe3O4,along with the increase in the thickness and hardness of oxide layer.The ablation of graphite particles is aggravated.(4)The pre-toxidation temperature on the tribological properties of the Cu-based brake pad were investigated.The results show that the formed oxides and ablated graphite result in high friction coefficient and wear rate.Due to the loss of plastic deformation ability,the dominant wear mechanism of brake pad transforms from delamination wear to abrasive wear with the increase of pre-oxidation temperature.(5)Cu-based friction materials reinforced with AlCoCrFeNi high-entropy alloy contents of 0～40 wt%were prepared,and the effects of AlCoCrFeNi HEA content on microstructure and braking propertiese were investigated.The results show that appropriate amount of AlCoCrFeNi enhanced the friction materials obviously.With the AlCoCrFeNi content increasing,the amount of interfaces between AlCoCrFeNi particles and Cu matrix increase and the bending strength of the materials decreases,owing to the formation of oxides and insufficient sintering.(6)The Effects of AlCoCrFeNi high-entropy alloy content on braking performance of Cu-based friction materials were investigated.The friction coefficient and absorbing power of Cu-based friction materials can be significantly improved by adding AlCoCrFeNi particles.However,the improvement in their wear resistance is not significant.The spalling pits and oxides are detected on the worn surfaces.The dominant wear mechanisms of Cu-based friction materials are spalling and oxidation.